Part with very high mechanical properties from a rolled coated sheet

ABSTRACT

The present invention provides a steel part. The steel part includes a steel precoated with a zinc-based alloy including, the contents being expressed by weight, from 0.5 to 2.5% aluminum and, optionally, one or more elements chosen from the group of: Pb≤0.003%; Sb≤0.003%; Bi≤0.003%; 0.002%.≤Si≤0.070%; La&lt;0.05%; and Ce&lt;0.05%. A balance of the precoat includes zinc and inevitable impurities. The steel part also includes a compound formed by at least one heat treatment for alloying between the steel and the precoat. The compound includes over more than 90% of its thickness, at least one Fe/Zn-based phase, the iron weight content of which is equal to 65% or higher and the Fe/Zn ratio of which is between 1.9 and 4. A structural or safety part is also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. application Ser. No. 12/091,635 filed onSep. 26, 2008 which was a national stage of PCT/FR2006/002316 filed onOct. 12, 2006 which claims priority to PCT/FR2005/002689 filed on Oct.27, 2005.

The invention relates to the manufacture of hot-rolled or cold-rolledcoated steel parts exhibiting high mechanical strength and goodcorrosion resistance.

BACKGROUND

For some applications, it is desired to produce steel parts that combinehigh mechanical strength, good impact strength and good corrosionresistance. This type of combination is particularly desirable in theautomobile industry, in which the objective is to produce significantlylighter vehicles. This may in particular be achieved by using parts madeof steels having very high mechanical properties, the microstructure ofwhich is martensitic or bainitic-martensitic. Anti-intrusion, structuralor safety parts of motor vehicles, such as fender cross-members, door orcenter pillar reinforcements and wheel arms, require for example theabovementioned qualities.

Patent FR20004427 discloses a manufacturing process in which a rolledsteel sheet is provided with a metal precoat consisting of zinc or azinc-based alloy, the steel possessing, for example, a tensile strengthof around 500 MPa. The sheet is then cut to obtain a blank, which issubjected to a heat treatment for the purpose of forming an alloyedcompound on the surface and of hot-stamping the blank. Next, this blankis cooled under conditions suitable for giving the steel a highhardness. By starting with a steel having an initial strength of 500MPa, parts with a mechanical strength of greater than 1500 MPa are forexample obtained. The alloyed compound formed by interdiffusion of theprecoat and the steel during the heat treatment thus provides protectionagainst corrosion and decarburation, and provides a high-temperaturelubrication function, which allows the lifetime of hot-drawing tools tobe increased.

In comparison with a hot-stamping process carried out on bare parts,that is to say those with no precoat, the presence of the compoundprovides protection against decarburation during in-furnace heating. Italso dispenses with the need to shotpeen or sandblast the partssubsequently, in order to remove the irregular surface layer that formsby oxidation in the furnace.

However, limitations may be encountered when carrying out this processin certain applications that require particular properties of thecoating formed by alloying:

the hot-stamped parts may include regions of pronounced concavity. Giventhe difference in hot hardness and rheology between the base steel andthe coating, the phenomenon of coating indentation in the base steel maybe encountered, in particular in highly deformed regions. In the case ofparts that are mechanically highly stressed, it is desirable to avoidthese indentations, which are potential defect initiation zones;

during the heat treatment resulting in the alloying between the steeland the precoat, iron-rich Fe/Zn phases are nucleated and zinc nearthese nucleation sites undergoes diffusion. This diffusion createsvacancies, possibly resulting in the creation of compactness defects ata microscopic level. The most favorable manufacturing conditions aretherefore sought in order to reduce or eliminate these compactnessdefects in the coating;

it is also sought to minimize tool wear during forming operations, whichmay be relatively pronounced depending on the coating. It has been foundthat coatings with a high roughness are disadvantageous with regard totool integrity. It is therefore endeavored to obtain conditions thatreduce the roughness of this coating; and

it is also sought to obtain a regular surface appearance of the coatingafter the alloying heat treatment, when the parts are intended topossibly undergo a subsequent painting operation or are to be used asvisible parts.

BRIEF SUMMARY

In particular, an object of the present invention is to avoid theappearance of surface crazing after the heat treatment. Such a visualdefect in the coating is characterized by the juxtaposition of cells,generally having a size of a few millimeters, separated by boundaries.Within any one cell, the thickness of the coating is approximatelyconstant, whereas the thickness of the coating is irregular at cellboundaries.

An object of the present invention provides a process for manufacturinghot-rolled or cold-rolled steel parts precoated with a zinc-based alloy,which includes an alloying treatment step, the coating obtained afteralloying having good compactness at the same time as high resistance tocrazing and a roughness associated with satisfactory longevity of theforming tool. It is also desired to provide a process that does notresult in indentation defects.

The present invention provides a steel part coated with a compoundconsisting, over more than 90% of its thickness, of at least oneFe/Zn-based phase, the Fe weight content of which is equal to 65% orhigher and the Fe/Zn ratio of which is between 1.9 and 4, the compoundbeing formed by at least one heat treatment for alloying between thesteel and a precoat, the precoat being a zinc-based alloy comprising,the contents being expressed by weight, between 0.5 and 2.5% aluminumand, optionally, one or more elements chosen from: Pb≤0.003%; Sb≤0.003%;Bi≤0.003%; 0.002%≤Si≤0.070%; La<0.05%; Ce<0.05%, the balance consistingof zinc and inevitable impurities.

Preferably, the precoat is an alloy the aluminum content of which is notless than 0.5% but not more than 0.7% by weight.

According to a preferred embodiment, the precoat is an alloy thealuminum content of which is greater than 0.7% but not more than 0.8% byweight.

Also preferably, the precoat is an alloy the aluminum content of whichis greater than 0.8% but not more than 2.5% by weight.

Preferably, the composition of the steel comprises, the contents beingexpressed by weight: 0.15%≤C≤0.5%; 0.5%≤Mn≤3%; 0.1%≤Si≤0.5%;0.01%≤Cr≤1%; Ti≤0.2%; Al≤0.1%; S≤0.05%; P≤0.1%; 0.0005%≤B≤0.010%, thebalance of the composition consisting of iron and inevitable impuritiesresulting from the smelting.

According to a preferred embodiment, the composition of the steelcomprises, the contents being expressed by weight: 0.15%≤C≤0.25%;0.8%≤Mn≤1.5%; 0.1%≤Si≤0.35%; 0.01%≤Cr≤0.3%; Ti≤0.1%; Al≤0.1%; S≤0.05%;P≤0.1%; 0.002%≤B≤0.005%, the balance of the composition consisting ofiron and inevitable impurities resulting from the smelting.

The subject of the invention is also a process for manufacturing acoated steel part, comprising the steps according to which:

a hot-rolled or cold-rolled steel sheet is provided;

the sheet is coated with a metal precoat being formed by a zinc-basedalloy comprising, the contents being expressed by weight, between 0.5and 2.5% aluminum and, optionally, one or more elements chosen from:Pb≤0.003%; Sb≤0.003%; Bi≤0.003%; 0.002%≤Si≤0.070%; La<0.05%; Ce<0.05%,the balance consisting of zinc and inevitable impurities, a heatpretreatment is optionally carried out, the sheet is cut in order toobtain a part, the part is heated so as to form, by alloying between thesteel and the precoat, an alloyed coating consisting, over more than 90%of its thickness, of at least one Fe/Zn-based phase, the Fe weightcontent of which is equal to 65% or higher and the Fe/Zn ratio of whichis between 1.9 and 4, and so as to give the steel a partially orcompletely austenitic structure;

the part undergoes hot deformation and the part is cooled underconditions suitable for giving the steel part the intended mechanicalproperties.

According to a preferred embodiment, the precoat is an alloy thealuminum content of which is not less than 0.5% but not more than 0.7%by weight.

Also preferably, the precoat is an alloy the aluminum content of whichis greater than 0.7% but not more than 0.8% by weight.

Preferably, the precoat is an alloy the aluminum content of which isgreater than 0.8% but not more than 2.5% by weight.

According to a preferred embodiment, a hot-rolled or cold-rolled steelsheet is provided, the composition of which comprises, the contentsbeing expressed by weight: 0.15%≤C≤0.5%; 0.5%≤Mn≤3%; 0.1%≤Si≤0.5%;0.01%≤Cr≤1%; Ti≤0.2%; Al≤0.1%; S≤0.05%; P≤0.1%; 0.0005%≤B≤0.010%, thebalance of the composition consisting of iron and inevitable impuritiesresulting from the smelting.

Also preferably, a hot-rolled or cold-rolled steel sheet is provided,the composition of which comprises, the contents being expressed byweight: 0.15%≤C≤0.25%; 0.8%≤Mn≤1.5%; 0.1%≤Si≤0.35%; 0.01%≤Cr≤0.3%;Ti≤0.1%; Al≤0.1%; S≤0.05%; P≤0.1%; 0.002%≤B≤0.005%, the balance of thecomposition consisting of iron and inevitable impurities resulting fromthe smelting.

According to one preferred embodiment, the heat pretreatment comprisesheating up to a temperature ranging from 450° C. to 520° C. for a soaktime thereof ranging from 2 to 10 minutes.

Preferably, in order to achieve the alloying and to give the steel apartially or completely austenitic structure, the heating is carried outat a temperature between Ac1 and Ac3+100° C., the duration of the soakat said temperature being not less than 20 s.

The subject of the invention is also the use of a part described above,or manufactured according to one of the variants described above, forthe manufacture of structural or safety parts for a terrestrial motorvehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent overthe course of the description given below by way of example and withreference to the following appended figures:

FIG. 1 shows the variation of an index that characterizes the quality ofthe coating as a function of the aluminum content of the zinc-basedprecoat;

FIG. 2 is a surface view of crazing observed on the surface of a steelcoated using a manufacturing process not in accordance with theinvention; and

FIG. 3 is a microstructural view in cross section of a steel sheethaving a coating according to the invention.

DETAILED DESCRIPTION

Compared with a precoat obtained from pure zinc, the inventors havesurprisingly discovered that the quality of the coating formed after aheat treatment for alloying between the base steel and the precoat isconsiderably improved when the precoat consists of a zinc-based alloycontaining a particular amount of aluminum. FIG. 1 shows the variationof an index characterizing the quality of the coating as a function ofthe aluminum content of the zinc-based precoat. This index takes intoaccount the compactness, roughness and crazing-resistance properties ofthe coating. The rating for this index goes from 0 to 10 (10=very goodcompactness, roughness and crazing-resistance properties; 0=verymediocre behavior).

When the aluminum weight content of the precoat is less than 0.5%, thecompactness of the alloyed coating formed is mediocre, the coatinghaving pores of variable size that developed during the alloyingtreatment. In addition, under these conditions the presence of a highlydeveloped crazing network is revealed.

When the aluminum content is greater than 2.5%, the quality of thecoating decreases substantially owing to an increase in roughness.

When the aluminum content of the zinc-based precoat is between 0.5 and0.7%, the coating has an advantageous combination, particularly withregard to the roughness and crazing-resistance properties. Theseproperties are further improved when the aluminum content is greaterthan 0.7% but does not exceed 0.8%.

The optimum combination of compactness, wear resistance and crazingresistance is obtained when the aluminum content of the zinc bath isgreater than 0.8% but no more than 2.5%.

The zinc-based precoat may be deposited on the base steel by ahot-dipping process, by electrodeposition, by a vacuum depositionprocess or by any other process. The deposition is preferably carriedout continuously. Apart from aluminum, the zinc-based precoat mayoptionally contain one or more elements from:

lead, antimony and bismuth, the weight content of each of these threeelements not exceeding 0.003% in order to avoid the spangling effect inthe case of hot-dipped coatings;

silicon, with a weight content not less than 0.002%, makes it possibleto avoid the formation of an excessively large Fe_(x)Al_(y) interfaciallayer. However, when the silicon content is greater than 0.070%, drossis formed in the case of hot-dipped coatings; and

lanthanum and cerium, in an amount not exceeding 0.05%, favorwettability of the surface with respect to the zinc bath.

The zinc-based precoat may also contain inevitable impurities, such asfor example cadmium, tin or copper. When the precoat is formed by ahot-dip process, iron and manganese may thus be especially present asimpurities.

Advantageously, the base steel on which the precoat is deposited has thefollowing composition by weight:

a carbon content of between 0.15 and 0.5%, and preferably between 0.15and 0.25% by weight. This element plays a major role in thehardenability and in the mechanical strength obtained after the coolingthat follows the austenization and alloying treatment. Below a contentof 0.15% by weight, the hardenability is however too low and thestrength properties are insufficient. In contrast, above a content of0.5% by weight, risk of forming defects is greater during hardening,particularly in the case of the thickest parts. A carbon content ofbetween 0.15 and 0.25% makes it possible to obtain a strength of betweenapproximately 1250 and 1650 MPa;

manganese, apart from its deoxidizing role, also has a major effect onthe hardenability, in particular when its weight content is at least0.5% and preferably 0.8%. However, too large an amount (3% by weight orpreferably 1.5%) results in the risk of excessive segregation;

the silicon content of the steel must be between 0.1 and 0.5% by weight,and preferably between 0.1 and 0.35% by weight. Apart from its role indeoxidation of the liquid steel, this element contributes to thehardening of the steel, but its content must however be limited in orderto avoid excessive formation of oxides and to promote coatability;

chromium, above a content greater than 0.01%, increases thehardenability and contributes to achieving a high strength after thehot-forming operation, in the various portions of the part after thecooling following the alloying and austenization heat treatment. Above acontent of 1% (preferably 0.3%), the contribution of chromium toachieving this uniformity in mechanical properties is saturated;

aluminum is an element that promotes deoxidation and precipitation ofnitrogen. In an amount greater than 0.1% by weight, it forms coarsealuminates during production, which encourages the content of aluminumto be limited to this value;

sulfur and phosphorus in excessive quantities result in increasedbrittleness. This is why it is preferable to limit their respectivecontents to 0.05 and 0.1% by weight;

boron, the content of which must be between 0.0005 and 0.010% by weight,and preferably between 0.002 and 0.005% by weight, is an element thatplays a major role in hardenability. Below a 0.0005% content, nosatisfactory hardenability effect is obtained. The full effect isobtained for a content of 0.002%. The maximum boron content must be lessthan 0.010%, and preferably 0.005%, in order not to degrade thetoughness; and

titanium has a high affinity for nitrogen, and therefore helps toprotect boron so that this element is in free form in order to house itsfull effect on hardenability. Above 0.2%, and more particularly 0.1%,there is however a risk of forming coarse titanium nitrides in theliquid steel, which have a deleterious effect on toughness.

In the process according to the invention, a hot-rolled or cold-rolledsheet of steel having the composition presented above is provided andprecoated with a zinc-based alloy having the composition also presentedabove. Before or after heat treatment, the sheet is cut in order toobtain a part. This part is then heated in order to carry out, jointly:

an alloying treatment so as to form a coating consisting, over more than90% of its thickness, of at least one Fe/Zn-based phase, the Fe weightcontent of which is equal to 65% or higher and the Fe/Zn ratio of whichis between 1.9 and 4. During the alloying reaction, the elements of thesteel sheet, especially iron, manganese and silicon, diffuse into thecoating. Certain elements of the precoating, especially zinc andaluminum, also diffuse; and

an austenization of the base steel, this austenization possibly beingpartial or complete. Advantageously, the heating in a furnace is carriedout in such a way that the part reaches a temperature between Ac1 andAc3+100° C. Ac1 and Ac3 denote the austenitic transformation start andend temperatures respectively. According to the invention, the soak timeat this temperature is not less than 20 s so as to make the temperaturein the various points of the part uniform. The hot-forming operation isthen carried out on the part, this operation being favored by thereduction in flow stress and the increase in ductility of the steel withtemperature. Starting from the partially or completely austeniticstructure, the part will then be cooled under appropriate conditions soas to give the intended mechanical properties to the part. Inparticular, the part may be kept within a tool during the cooling, thetool itself possibly being cooled in order to favor extraction of theheat. To obtain high mechanical properties, it will be preferable toobtain martensitic or bainitic-martensitic microstructures.

Optionally, a heat pretreatment may be carried out after theabovementioned precoating step. This heat pretreatment comprises heatingup to a temperature ranging from 450.degree. C. to 520.degree. C. for atime ranging from 2 to 10 minutes. This heat pretreatment increases thecompactness of the coating formed after the combinedalloying/austenization treatment, and also the crack resistance of thiscoating. It has also been found that this heat pretreatment favors theformation of coatings consisting, over more than 90% of their thickness,of two iron-rich phases, the iron weight content of which is equal to65% or higher and the Fe/Zn ratio of which is between 1.9 and 4. In theabsence of pretreatment, the coatings tend to consist of a single,iron-rich, phase. Without wishing to be tied by one theory, it isbelieved that this pretreatment modifies the interface between the steeland the precoat, and therefore the diffusion phenomena which occurduring the subsequent alloying treatment.

As examples, cold-rolled steel sheets with a thickness ranging from 1.3to 1.6 mm were considered, these having the following composition byweight:

carbon: 0.22%;

manganese: 1.3%;

silicon: 0.30%;

phosphorus<0.010%;

sulfur: 0.005%;

chromium: 0.18%;

titanium: 0.025%;

aluminum: 0.050%; and

B: 0.003%.

The steel sheets were precoated by hot-dipping in a bath based on zinccontaining aluminum in an amount ranging up to 5%, lead, antimony andbismuth, these three elements in an amount of less than 0.003%, and alsoiron, as inevitable residual element, in an amount of less than 0.020%.Pure zinc precoats were also deposited by electrodeposition. In the caseof hot-dipped coatings, the thickness of the precoat was about 10 to 20microns, while in the case of electrodeposited coatings, the thicknesswas around 10 microns.

Some of the sheets underwent an alloying heat pretreatment between 470and 520° C. for a time ranging from 2 to 10 minutes. The sheets werethen cut to obtain parts.

These parts were then heated up to a temperature of 930° C. (i.e.Ac3+70° C.) and soaked for 3 minutes at this temperature. The heatingtime, including the temperature rise time and the soak time at 930° C.,was 10 minutes. These conditions resulted in complete austenitictransformation of the base steel. During this heat-up and soak phase, itwas found that the zinc-based precoat formed, over more than 90% of itsthickness, one or more Fe/Zn phases, the iron weight content of whichwas 65% or higher and the Fe/Zn ratio of which was between 1.9 and 4, byan alloying reaction between the base steel and the zinc-based precoat.This alloyed coating having a high melting point and a high hardnessexhibits great corrosion resistance and prevents the subjacent basesteel from being oxidized and decarburized during and after the heatingphase.

After the 930° C. heating phase, the parts were subjected to a 5% hotdeformation.

Subsequent cooling in air resulted in a bainitic-martensitic structure.The mechanical strength obtained after such a treatment was greater than750 MPa.

The alloyed coatings were then characterized by the followingtechniques:

micrograph sections were used to assess the compactness of the coatings,and also the presence of any indentation thereof within the base sheetin certain hot-deformed zones;

a visual observation and measurements carried out on a roughness meterenabled the roughness parameter Ra to be quantified and the crazing ofthe coatings after heat treatment and deformation, and also the wearresistance of the tools, to be evaluated; and

observations using a scanning electron microscope in phase contrast modeenabled the phases present in the coatings to be identified.

The results of these observations are the following:

under the conditions according to the invention, the coating formed byalloying consists of iron-rich Fe/Zn phases, the iron weight content ofwhich is equal to 65% or higher and the Fe/Zn ratio of which is between1.9 and 4, over more than 90% of its thickness. The micrograph shown inFIG. 3, obtained by scanning electron microscopy, illustrates an exampleaccording to the invention: the alloyed coating consists mostly over itsthickness of two phases: a very pale phase of mean composition: 70%Fe/27% Zn/1% Al/0.4% Si and a phase of light gray appearance comprising76% Fe/22% Zn/1% Al/0.5% Si. The presence of manganese in smalleramounts may be noted. The presence of silicon and manganese, and ofcourse the presence of iron, bears witness to the diffusion of the basemetal into the precoat during the alloying/austenization treatment. Afew rare residual pores (dark regions) are also present. At theoutermost surface of the specimen, the presence of a higher zinc contentmay be noted, which reinforces the corrosion protection;

when the aluminum content is less than 0.5% in the precoat, thecompactness of the alloyed coating formed is mediocre, the coatinghaving many relatively well-developed pores. Under these conditions, thepresence of a highly pronounced surface crazing network is alsorevealed. FIG. 2 shows an example of such crazing for an aluminumcontent of 0.1%, that is to say outside the conditions of the invention;

when the aluminum content is greater than 2.5% in the precoat, theroughness increases substantially, going from Ra=1.3 microns to Ra=3microns; and

when the aluminum content of the zinc-based precoat is between 0.5 and2.5%, the coating exhibits a very good combination of compactness, lowroughness and absence of crazing. It is also noted that there is noindentation of the coating into the base steel during the hotdeformation, even in the regions of pronounced concavity. In addition,when the aluminum content is greater than 0.7% and preferably 0.8%, theresistance to the occurrence of crazing is at its highest level.

Thus, the invention makes it possible to manufacture coated parts havinghigh properties, the metal coating having a particularly favorablecombination of compactness, low roughness, absence of crazing andindentation resistance. The maximum strength of the parts may be adaptedto the intended use according to the composition of the steel, inparticular its carbon content and its manganese, chromium and boroncontent.

These parts will be profitably used for the manufacture of safety parts,and especially anti-intrusion or substructure parts, strengthening bars,and center pillars, for the construction of motor vehicles.

What is claimed is:
 1. A steel part comprising: a steel substrate; and acompound formed by at least one heat treatment alloying the steelsubstrate and a precoat of a zinc-based alloy including, the contents ofthe precoat being expressed by weight, greater than 0.8% but not morethan 2.5% aluminum and, optionally, one or more elements chosen from thegroup consisting of: Pb≤0.003%; Sb≤0.003%; Bi≤0.003%; 0.002%≤Si≤0.070%;La<0.05%; and Ce<0.05%; a balance being zinc and inevitable impurities;and a coating comprising said compound, the coating having a thicknessextending from the steel substrate, over more than 90% of the thickness,at least one Fe/Zn-based phase, the iron weight content of which isequal to 65% or higher and the Fe/Zn ratio of which is between 1.9 and4.
 2. The steel part as recited in claim 1, wherein a composition of thesteel comprises, the contents being expressed by weight: 0.15%≤C≤0.5%;0.5%≤Mn≤3%; 0.1%≤Si≤0.5%; 0.01%≤Cr≤1%; Ti≤0.2%; Al≤0.1%; S≤0.05%;P≤0.1%; and 0.0005%≤B≤0.010%, a balance of the composition includingiron and inevitable impurities resulting from the smelting.
 3. The steelpart as claimed in claim 2, wherein the composition of the steelcomprises, the contents being expressed by weight: 0.15%≤C≤0.25%;0.8%≤Mn≤1.5%; 0.1%≤Si≤0.35%; 0.01%≤Cr≤0.3%; Ti≤0.1%; Al≤0.1%; S≤0.05%;P≤0.1%; and 0.002%≤B≤0.005%; a balance of the composition including ironand inevitable impurities resulting from the smelting.
 4. A structuralor safety part for a terrestrial motor vehicle comprising: a steel partas recited in claim
 1. 5. The steel part as claimed in claim 1, whereinthe precoat of a zinc-based alloy includes one or more elements chosenfrom the group consisting of: Pb≤0.003%; Sb≤0.003%; and Bi≤0.003%. 6.The steel part as claimed in claim 1, wherein the precoat of azinc-based alloy includes 0.002%≤Si≤0.070%.
 7. The steel part as claimedin claim 1, wherein the precoat of a zinc-based alloy includes one ormore elements chosen from the group consisting of La<0.05%; andCe<0.05%.
 8. The steel part as claimed in claim 1, wherein the at leastone Fe/Zn-based phase includes a first phase and a second phase eachwith the iron weight content equal to 65% or higher and the Fe/Zn ratiobetween 1.9 and 4, but with different iron weights.
 9. The steel part asclaimed in claim 1, wherein the at least one Fe/Zn-based phase includesa first phase and a second phase each with the iron weight content equalto 65% or higher and the Fe/Zn ratio between 1.9 and 4, but withdifferent zinc weights.
 10. The steel part as claimed in claim 1,wherein the at least one Fe/Zn-based phase includes a first phase and asecond phase each with the iron weight content equal to 65% or higherand the Fe/Zn ratio between 1.9 and 4, but with different iron and zincweights.
 11. The steel part as claimed in claim 1, wherein the at leastone Fe/Zn-based phase includes manganese.
 12. The steel part as claimedin claim 1, wherein the base steel has a martensitic microstructure. 13.The steel part as claimed in claim 1, wherein the base steel isbainitic-martensitic microstructure.
 14. The steel part as claimed inclaim 1, wherein the steel part has a mechanical strength greater than750 Mpa.